The invention relates to digital imaging devices and more particularly to pixel layouts or configurations.
Image sensing devices are the light detecting component in digital imaging systems, such as for example, digital cameras, scanners and copiers. An image sensing device, such as a camera, uses light to capture an image at a semiconductor-based chip. The chip replaces film in traditional film-based systems. In a camera, an image sensing devices is configured, in its simplest form, to capture a monochrome or a color image by way of semiconductor devices such as transistors, capacitors, and photodiodes. In one example, the image sensing device is a chip made up of a number of pixels, each pixel is sensitive to light.
A pixel contains a photosensitive structure, such as a photodiode, and other pixel circuitry. The photosensitive structure is the portion of the pixel that respond to light. In a typical operation, a pixel circuit having a photodiode is charged to a predetermined voltage, the photodiode is exposed to light, and the pixel circuit discharges its stored energy depending on the intensity of the light exposure. A three-transistor pixel circuit for example, consists of a reset transistor, a row transistor, a source follower transistor, and a photodiode. A shift register or decoder external to the pixel circuit generates a signal that inverts the reset transistor and the row transistor (through a bit line) to drive the circuit and read out a predetermined value (e.g., Vcc) and the light signal. The difference between the predetermined voltage and the light signal is a measure of the intensity of the light striking the photodiode.
Pixel circuits may be arranged in pixel arrays, such as, for example, several rows and columns of pixel circuits. The individual pixel circuits may be coupled through word and bit lines. The reset signal generated by the shift register or decoder may be global (i.e., entire array) or may be limited to a single pixel or row of pixels. The bit line signals a row to discharge its signal along the word line to a collector such as an analog-to-digital (A/D) converter.
Pixel circuits, such as the three-transistor pixel circuit described above are formed, in one embodiment, in and on a semiconductor-based chip. One goal in forming a pixel circuit or an array of pixel circuits is to maximize the photosensitive area of the circuit and minimize the amount of area required for the additional pixel circuitry.
The trend in digital imaging is towards higher and higher image resolution. One way this is achieved is by reducing the size of pixel circuits. The trend is particularly pushed by charge coupled device (CCD) manufacturers, as one of the main advantages of a CCD manufacturer is a small pixel circuit. One way that CMOS-based image sensors may stay competitive with their CCD-based counterparts is by reducing the pixel size to a size similar to a CCD pixel with comparative sensitivity.
An image sensor circuit is disclosed. In one aspect, the image sensor circuit includes a set of pixels associated with a pixel output node and a reset circuit associated with the set of pixels to set each pixel to a predetermined value.